This invention relates to titanium aluminide/fiber composite materials. In particular, this invention relates to a method for fabricating such composite materials.
In recent years, material requirements for advanced aerospace applications have increased dramatically as performance demands have escalated. As a result, mechanical properties of monolithic metallic materials such as titanium alloys often have been insufficient to meet these demands. Attempts have been made to enhance the performance of titanium by reinforcement with high strength/high stiffness filaments or fibers.
Titanium matrix composites have for quite some time exhibited enhanced stiffness properties which closely approach rule-of-mixtures (ROM) values. However, with few exceptions, both tensile and fatigue strengths are well below ROM levels and are generally very inconsistent.
These titanium matrix composites are typically fabricated by superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers. Several high strength/high stiffness filaments or fibers for reinforcing titanium alloys are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron, titanium boride-coated silicon carbide and silicon-coated silicon carbide. Under superplastic conditions, which involve the simultaneous application of pressure and elevated temperature for a period of time, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fiber. The thus-contacting layers of matrix material bond together by a phenomenon known as diffusion bonding.
Metal matrix composites made from conventional titanium alloys such as Ti-6Al-4V or Ti-15V-3Cr-3Al-3Sn can operate at temperatures of about 400.degree. to 1000.degree. F. Above 1000.degree. F. there is a need for matrix alloys with much higher resistance to high temperature deformation and oxidation.
Titanium aluminides based on the ordered alpha-2 Ti.sub.3 Al phase are currently considered to be one of the most promising group of alloys for this purpose. However, the Ti.sub.3 Al ordered phase is very brittle at lower temperatures and has low resistance to cracking under cyclic thermal conditions. Consequently, groups of alloys based on the Ti.sub.3 Al phase modified with beta stabilizing elements such as Nb, Mo and V have been developed. These elements can impart beta phase into the alpha-2 matrix, which results in improved room temperature ductility and resistance to thermal cycling. However, these benefits are accompanied by decreases in high temperature properties. With regard to the beta stabilizer Nb, it is generally accepted in the art that a maximum of about 11 atomic percent (21 wt %) Nb provides an optimum balance of low and high temperature properties in unreinforced matrices.
Titanium matrix composites have not reached their full potential, at least in part, because of problems associated with instabilities at the fiber-matrix interface. At the time of high temperature bonding a reaction can occur at the fiber-matrix interfaces, giving rise to what is called a reaction zone. The compounds formed in the reaction zone may include reaction products such as TiSi, Ti.sub.5 Si, TiC, TiB and TiB.sub.2 when using the previously mentioned fibers. The thickness of the reaction zone increases with increasing time and with increasing temperature of bonding. The reaction zone surrounding a filament introduces sites for easy crack initiation and propagation within the composite, which can operate in addition to existing sites introduced by the original distribution of defects in the filaments It is well established that mechanical properties of metal matrix composites are influenced by the reaction zone, and that, in general, these properties are degraded in proportion to the thickness of the reaction zone.
In metal matrix composites fabricated from the ordered alloys of Ti.sub.3 Al+Nb, the problem of reaction products formed at the metal/fiber interface becomes especially acute, because Nb is depleted from the matrix in the vicinity of the fiber. The thus-beta depleted zone surrounding the fiber is essentially a pure, ordered alpha-2 region with the inherent low temperature brittleness and the low resistance to thermal cycling. The resistance to thermal cycling is generally so low that the material cracks during the thermal cycle associated with fabrication of a metal matrix composite.
Accordingly, it is an object of the present invention to provide a method for fabricating an improved titanium aluminide metal matrix composite.
It is another object of this invention to provide an improved titanium aluminide metal matrix composite.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.